In the field of plastic molding, precision processing technology for molding dies has been established, and the mass production of optical elements having minute shapes such as a diffraction grating has been achieved. In this case, a die is manufactured in the following manner: the surface of a base material made of stainless steel is subjected to electroless Ni—P plating, and this plated layer is then precisely processed by a diamond turning tool.
However, if similar dies are applied to press forming of glass, there arises a problem that cracks are caused in the electroless Ni—P plated layer. This phenomenon results from a press forming temperature. That is, the Ni—P plated layer assumes an amorphous structure in a plated state, but starts crystallization when heated to about 270° C. or more. At this point, a volumetric shrinkage is caused in the plated layer, and tensile stress is produced so that the cracks are caused in the plated layer.
As countermeasures against this problem, in Jpn. Pat. Appln. KOKAI Publication No. 11-157852, a base material having a thermal expansion coefficient of 10×10−6 to 16×10−6 (K−1) is selected, and a heat treatment is carried out at 400 to 500° C. after plating. However, even if the thermal expansion coefficient of the base material is adapted to that of the Ni—P plated layer, a volumetric shrinkage resulting from the crystallization is caused only in the plated layer during the heat treatment, so that the high tensile stress may be produced in the plated layer leading to cracks.